This invention relates to the technique of ultrasonic echoscopy of objects and in particular to an extension of known techniques of ultrasonic echoscopy to provide more useful information concerning the examined objects. It is particularly, but not solely, directed to the more effective acquisition of data in medical diagnosis utilising this technique.
Ultrasonic echoscopy provides information about an examined object which may be displayed in the form of an ultrasonic echogram. Such an echogram consists of a display of acoustic impedance discontinuities or reflecting surfaces in the object. It is obtained by directing a short pulse of ultrasonic energy, typically in the 1-30 MHz frequency range, into the examined object where any acoustic impedance discontinuities in the object reflect and return some of the energy in the form of an echo. This echo is received, converted into an electric signal and displayed as an echogram on a cathode ray oscilloscope, a film, a chart or the like.
The echogram may consitutute either a one dimensional or a two dimensional representation and in both cases the information is contained in the position and magnitude of the echo displayed. In a one dimensional display, the position along a base line is used to indicate the distance to the reflecting surface whilst the magnitude of the echo is displayed, for example, as a deflection of the base line or as an intensity change. In a two dimensional display, the position along a base line is used to indicate the distance to the reflecting surface as in a one dimensional display, and the direction of the base line is used to represent the direction of propagation of the acoustic energy. The two dimensional display is obtained by changing this direction of propogation of the acoustic energy and by instituting a similar but not necessarily identical movement of the base line of the display. The magnitude of the echo is displayed as for a one dimensional display; for example, as a deflection of the base line or as an intensity change.
The technique of ultrasonic echoscopy is used in medical diagnosis to obtain information about the anatomy of patients. The application of this technique is now widely investigated and is described, for example, by D. E. Robinson in Proceeding of the Institution of Radio and Electronics Engineers Australia, Vol. 31, No. 11, pages 385-392, Nov., 1970: "The Application of Ultrasound in Medical Diagnosis." As pointed out in this article, ultrasonic echoscopy may be used to produce displays resembling anatomical cross-sections which have proved clinically useful when the desired information concerns physical dimensions, shapes of organs or structures or the like. Ultrasonic echography has proved of particular value as a diagnostic aid in the abdomen and pregnant uterus, eye, breast, brain, lung, kidney, liver and heart, these being areas of soft tissue with little bone and air. In general, the technique is considered to complement other techniques to provide a more complete picture of the patients condition, however particularly in pregnancies, ultrasonic echoscopy may be useful in place of X-rays where the latter may not give sufficient information or may be dangerous. In medical use, a pulse of ultrasonic energy is transmitted into a patient in a known direction and echoes are received from reflecting surfaces within the body. The time delay between a transmitted pulse and the received echo depends on the distance from the transmitter to the reflecting surface and the distance information so obtained may be displaced in a suitable way for interpretation and clinical use as a one dimensional range reading or as a two dimensional cross section as previously described.
This known system suffers from several disadvantages due to ambiguity in the received echo formation:-
i. The magnitude of the echo received is dependent on the reflectivity of the surface, its angulation with the beam and the scattering properties of the surface. When the reflector is sharply inclined to the beam no echo is returned.
ii. An echo at a particular range may be produced by a direct two-way travel in which case the indicated location of the reflecting surface is correct. However the pulse may be reflected several times within the examined object giving rise to false indication of a reflecting surface at a range at which none exists.
iii. Errors in the indicated positions of reflecting surfaces in the examined object due to local changes in velocity of propagation of the ultrasonic energy through the object are not detectable, nor is the velocity of propagation measurable.
It is a primary object of the present invention to provide an improved apparatus and method for the ultrasonic echoscopic examination of objects whereby more reliable and more useful information may be obtained concerning the examined objects. The improvement lies in a greatly extended capacity of the information acquisition and processing system which gives rise to one line of ultrasonic data on a two-dimensional display. This line with increased information may then be used in the same way as a simple pulse echo signal line to provide a two dimensional picture according to the current art as explained above.
According to this invention, there is provided apparatus for the ultrasonic examination of an object comprising means for transmitting pulses of ultrasonic energy into the object and means for receiving echoes of said pulses of ultrasonic energy reflected by acoustic impedance discontinuities within the object, wherein: said means for transmitting pulses comprises a central transducer adapted to transmit pulses along a single axis and a plurality of annular transducers positioned concentrically with said axis of the central transducer, and said means for receiving echoes comprises said central transducer, the central transducer being adapted to receive echoes of pulses transmitted into the object by each of said central transducer and said annular transducers which are reflected along said single axis thereof, said apparatus further comprising means for separately storing and then subsequently analysing said received echoes of pulses transmitted by each of said central transducer and said annular transducer to extract velocity, scattering, and multiple reflection information.
In an alternative embodiment, this invention provides apparatus for the ultrasonic examination of an object comprising means for transmitting pulses of ultrasonic energy into the object and means for receiving echoes of said pulses of ultrasonic energy reflected by acoustic impedance discontinuities within the object, wherein: said means for transmitting pulses comprises a central transducer adapted to transmit pulses along a single axis, and said means for receiving echoes comprises said central transducer and a plurality of annular transducers positioned concentrically with said axis of the central transducer, each of said central transducer and said plurality of transducers being adapted to receive reflected echoes of pulses transmitted into the object by said central transducer along said single axis thereof, said apparatus further comprising means for separately storing and then subsequently analyzing said received echoes of pulses transmitted by said central transducer to extract velocity, scattering, and multiple reflection information.
This invention also provides a method of ultrasonic examination of an object comprising the steps of transmitting pulses of ultrasonic energy into the object and receiving echoes of said pulses of ultrasonic energy reflected by acoustic impedance discontinuities within the object, wherein: said pulses are transmitted along a single axis by a central transducer and by a plurality of annular transducers positioned concentrically with said axis of the central transducer, and echoes of pulses transmitted into the object by each of said central transducer and said annular transducers reflected along said single axis are received by said central transducer, said method including separately storing and then subsequently analyzing said received echoes of pulses transmitted by each of said central transducer and said annular transducers to extract velocity, scattering and multiple reflection information.
In yet another embodiment, the invention provides a method of ultrasonic examination of an object comprising the steps of transmitting pulses of ultrasonic energy into the object and receiving echoes of said pulses of ultrasonic energy reflected by acoustic impedance discontinuities within the object, wherein: said pulses are transmitted along a single axis by a central transducer, and reflected echoes of pulses transmitted into the object by said central transducer along said single axis thereof are received by each of said central transducer and a plurality of annular transducers positioned concentrically with said axis of the central transducer, said method including separately storing and then subsequently analyzing said received echoes of pulses transmitted by said central transducer to extract velocity, scattering and multiple reflection information.
It will be appreciated from the above that in use of either embodiment of the apparatus of the present invention, a set of paths is generated along which the pulses are transmitted and echoes received. This system is thus quite different to an axial system using a single line of sight along the axis of symmetry and as the echoes received along each path in accordance with the present invention are kept separate for the purpose of analysis, in this respect the present system differs from annular array systems in the current art.
In a preferred aspect of the invention, the active area of the central transducer is a disc with focusing properties and the beam axis of the transducer lies along a line through the centre of the disc at right angles to it's surface. Methods of achieving focusing properties are well known. For instance a simple geometrical focus can be obtained as described in the scientific literature by G. Kossoff "Design of Narrow Beam Width Transducers" J. Acoust. Soc. Amer., 35, 6 (June 1963); 905-912. Alternatively an annular geometry may be used as described in U.S. Pat. No. 3,327,286. The active areas of the annular transducers are preferably arranged to be in the shape of thin annular rings whose axes lie along the same line as the axis of the central transducer. The diameters of the annular transducers are preferably of substantially larger diameter than the diameter of the disc of the central transducer and the width of each of the annular transducer is preferably small compared with the diameter of the disc of the central transducer. Typically, the diameters of the annular transducers are up to the same order of size as the distance from the transducer face to the examined object. By way of example, there may be from four to ten annular transducers in the apparatus.
The annular transducers may lie in the same plane as the central transducer, or they may be positioned to lie on the surface of a cone, sphere or other solid of revolution whose axis is the axis of the central transducer. In addition, each of the annular transducers may consist of a single element or alternatively it may be broken up into a number of separate segmental elements.
The central and annular transducers may consist of any type of electromechanical transducer.
In operation of the apparatus of the present invention in which the central transducer acts as the receiver, for one cycle of operation each transmitting transducer is pulsed in turn. Between pulses the receiver is activated and echoes received from along the receiver axis. The time delays of echoes received from each annular transducer are corrected to correspond to known distances along the receiver axis. A similar mode of operation is utilized where the central transducer acts as the transmitter, although if desired, echoes of a single pulse may be simultaneously received by the plurality of annular transducers. In either case, the sets of echoes corresponding to each beam path are kept separate for subsequent processing and analysis.
Processing of the information obtained in accordance with the present invention takes advantage of the fact that the ultrasound travels in a number of discreet paths which are not along the axis of the system and may be implemented by analogue or digital techniques and may consist of a number of steps:
i. By taking the echoes which are stored as a function of time delay and correcting them to account for the different path lengths travelled by the signals from the different transducers and for the velocity of propagation in the medium, a signal consisting of echoes as a function of distance along the axis is obtained. Echoes received from the same interface along different paths can then be directly compared, added, or otherwise combined.
ii. By adding echoes at common distances along the axis for all transmitted pulses in a cycle, inclined reflecting surfaces are displayed as echo indications as well as reflecting surfaces at right angles.
iii. By comparision of echo magnitude at a common distance for all pulses in a cycle, the scattering properties of a reflecting surface may be determined. Thus a measure of the reflectance and the roughness of the reflecting surface are obtained.
iv. By analysis of the difference in indicated distance between echoes which are apparently from the same reflecting surface of presence of multiple reflection artifacts is revealed.
v. By comparison of the distance difference and consideration of the outlines of anatomical structure an estimate of the velocity of propagation in areas of the medium may be obtained.